Device for controlling a mixture of two gases

ABSTRACT

A device for controlling a mixture of two gases comprises two nozzles mounted in the extension of one another and movable one with reference to the other, a ring, provided with an internal screw thread, being mounted in movable manner on a stationary sleeve enclosing one of the nozzles, the other one being mounted so as to be able to slide inside the said sleeve, at least one slot being provided in the sleeve, one driving member for the second nozzle, cooperating with such slot, being secured to the wall of the said nozzle and entering freely in the ring, so as to permit the said nozzle to move inside the device by making the ring rotate around its axis with reference to said sleeve.

United States Patent [191 Hermans 1 Mar. 26, 1974 DEVICE FOR CONTROLLINGA MIXTURE OF TWO GASES Fernand Victor Francois Hermans, Uccle, Belgium[73] Assignee: Le Four Industriel Belge, Uccle,

Belgium 22 Filed: Feb. 17, 1972 21 Appl. No.: 227,492

[75] Inventor:

[30] Foreign Application Priority Data Mar. 17, 1971 Belgium 764407 [52]US. Cl. 137/553, 48/180 P, 137/88, 137/604, 417/184, 417/191, 417/198,259/4 [51] Int. Cl. Fl6k 19/00 [58] Field of Search 48/180P; 137/88, 90,553, 137/604;259/4;4l7/183,184,191,198

[56] References Cited UNITED STATES PATENTS 2,178,898 11/1939 Schellinet a1. 417/183 3,643,688 2/1972 Meinert 137/604 1,583,363 5/1926Ostermann 417/184 2,888,191 5/1959 Neumann et a1. 417/183 X 2,946,2937/1960 Henshaw 417/183 3,031,127 4/1962 Duhaime et a1. 2,992,084 7/1961Schropp 3,689,237 9/1972 Stark et a1. 137/604 X Primary ExaminerRobertG. Nilson Attorney, Agent, or Firm--Sughrue, Rothwell, Mion, Zinn &Macpeak [5 7] ABSTRACT nozzle and entering freely in the ring, so as topermit the said nozzle to move inside the device by making the ringrotate around its axis with reference to said sleeve.

7 Claims, 3 Drawing Figures i :ll 1 v I: 1 I l I I I 5 "I i ll DEVICEFOR CONTROLLING A MIXTURE OF TWO GASES The present invention relates toa device for controlling a mixture of two gases, such as a comburent gasand a combustible gas, comprising a first nozzle with a convergingportion, such nozzle communicating with a first adduction pipe for oneof the said components of the mixture and in the rear of which is asecond nozzle coaxial with the first one, the second nozzle having adiverging portion, at least the entry of the latter being of largerinternal circular cross-section than the external circular cross-sectionof the first nozzle, a chamber wherein opens a second adduction pipe forthe other component of the said mixture, enclosing the outlet end of thefirst nozzle and communicating with the entry of the second nozzle,means being provided to vary along the common axis of the two nozzles,the relative position of the latter.

The aim of the invention is to provide an improved device of the saidtype enabling to achieve a strictly constant ratio, upstream of theequipment to be supplied, such as heating apparatus, between the twocomponents of the mixture and this for flows of this gas mixture varyingbetween fairly large limits.

For this purpose, in the device according to the invention, the meansprovided to vary, along the common axis of the twonozzles, the relativeposition of the latter comprise at least one ring, provided with aninternal screw thread, mounted in movable manner on a stationary sleeveenclosing one of the nozzles, the nozzle located inside the sleeve beingfitted in such a manner as to be able to slide inside the latter alongthe axis thereof; at least one slot, parallel to the said axis, passingright through the portion of the wall of the said sleeve covered by thesaid ring; a driving member for such nozzle co-operating with such slotand being secured to the inner wall of the nozzle and engaging freelythe inner face of the ring covering the slot, so as to permit the saidnozzle to move inside the device by making the ring rotate around itsaxis with reference to the sleeve.

Advantageously, the sleeve, comprises atleast two slots diametricallyopposite one another, parallel to the axis of the nozzle fitted insidethe sleeve, the said driving m'ember for such nozzle co-operating witheach one of the said slots.

According to a preferred embodiment of the invention, the first nozzleis assembled in the said sleeve, the second nozzle being stationary withreference to the latter.

According to a particular embodiment of the invention, a flange isprovided at the outlet of at least one of the nozzles, so as to permitestablishing close to such outlet, a stationary gas film.

Particular construction embodiments of the present invention will now bedescribed by way of non limitative examples, with reference to theaccompanying drawings in which:

F 1G. 1 shows diagrammatically a longitudinal crosssection partiallybroken away, of a particular embodiment of the invention.

FIG. 2 shows on a large scale a detail of the device shown in FIG. 1. I

FIG. 3 is an elevational view, partially broken away, of a detail of amodified form of the device shown in FIG. 1.

The device according to the invention, comprises a first nozzle 1, witha converging portion 2 and communicating with an adduction pipe 3 for afirst gas, and a second nozzle 4 coaxial with the first one and mounteddownstream of the latter.

The second nozzle 4 has a diverging portion, at least the entry 6 of thelatter being of larger internal circular cross-section than the externalcircular cross-section of the outlet 7 of the first nozzle 1, so as topermit engaging such outlet 7 in the entry 6 while maintaining inbetween them an annular space 8 for the passage of a gas originatingfrom a chamber 9 enclosing the outlet end of the first nozzle 1 andcommunicating with the entry 6 of the second nozzle 4. A secondadduction pipe 10 for a second gas opens into the said chamber 9.

A ring 11 with an internal helical thread 17 is freely threaded on astationary sleeve 12 enclosing the nozzle 1, the latter being fitted insuch a manner as to be able to translate inside the sleeve along thecommon axis 13.

Two slots 14 and 15, diametrically opposite, parallel to the said axis,pass right through the portion of the wall of the said sleeve covered bythe thread 17 of the ring 11. A driving member, provided by a stem 16,cooperates with each one of these slots 14 and 15. This member issecured by one of its ends to the outside wall of the nozzle and engagesfreely with its other end an annular slot 35 provided in the inner faceof the ring 11 and concentric with the latter, these two stems 16 beingalso diametrically opposite one another.

The movement of such nozzle 1 along its axis 13 is thus achieved bymaking the ring 11 rotate around its axis 13 with reference to thestationary sleeve 12.

The ring 11 is knurled on its outside face and provided with agraduation 18 showing the ratio of the two gases in terms of theposition of this ring of the sleeve 12.

As the position of the nozzle 1, for a given value of the ratio of thetwo gases, is a function of the density of the latter, a secondary ring19 is provided on the sleeve 12, partially beneath ring 11 and whichcarries on its outside face, parallel to its axis, a graduated scale 36.Ring 11 may be rotated around its axis in both directions. Thegraduation of the scale 36 in such that for one revolution of the ring11, the latter moves in the axial direction by one unit of thegraduation, so that the rim of the ring 11 located on the side of thering 19 shall coincide with the line mark according to the graduation36. The two graduated rings 11 and 19 thus determine a vernierpermitting measurement, in a very accurate manner, of the axial movementof the nozzle 1 and, consequently, also the ratio of the two gases.

The ring 19 may, just like the ring 11, rotate around its axis so as toenable a movement of the graduated scale 36 simultaneously with thegraduations 18 and thus ensure the accessibility of the reading of thescale, independently of the location of the device.

Furthermore, in order to allow for the types of gases used, the ring 19is able to slide, with reference to ring 11, on the sleeve 12 in thelongitudinal direction of the latter. This longitudinal motion must besuch that, in the case of using a combustible gas and a comburent gas, acomposition of gas mixture is obtained which is the neutral combustionor the theoretical basis composition when the line 0 of the graduation18 of the ring 11 is positioned facing the axial line of scale 36. Bychemical calibration of the stoichiometrical ratio of the combustiblegas comburent gas mixture, the line of the graduation 18 will thus beestablished facing the line 0" of the scale 36 on the ring 19 in such amanner as to obtain on one side of the line 0 of the ring 11 thegraduations corresponding to the reducing ratios and on the other sideof such line 0 the graduations corresponding to the oxidising ratios.

Between the nozzle 1 and the sleeve 12 is provided an annular gasket 20on either side of the slots 14 and 15.

These gaskets 20 are arrested in an annular housing 21 provided in theouter wall of the nozzle 1.

The outlet 7 of the nozzle 1 is provided with an outer circular flange22, shown on a larger scale in FIG. 2, which establishes a stationarygas film 39 on the outer wall of such nozzle 1, located in the space 8during the flow of the gas originating from the pipe 10 to the secondnozzle 4. The wall 23 of such flange 22, provided to establish the saidstationary gas film on the outer wall of the nozzle, forms with suchwall an angle equal to 90 at the most. The sleeve 12 is provided, at theend thereof directed towards the chamber 9, with an outer screw thread24, by means of which it is screwed on a hollow body 25 determining suchchamber 9.

The nozzle 4 which is stationary, is screwed in the said hollow body onthe side facing that one in which is screwed the sleeve 12.

The entry 6 of the nozzle 4 is converging and is formed by two conicalfrustums 27 and 28 placed end to end. The taper of the first frustum 27substantially exceeds that of the other frustum 28, which is of ratherlow taper. The conical frustrum 28 is connected to the said divergingportion by means of a cylindrical portion 29 providing a narrowing inthe nozzle 4.

The converging portion 2 of the nozzle 1 is extended by a cylindricalportion 30 as far as the outlet of the said nozzle, this cylindricalportion being coaxial with the nozzle 4 so as to achieve a laminar flowof the gas at the outlet of the nozzle 1. On the other hand, a pressureregulator 31 connected to a pipe 32 branched off the outlet 33 of thenozzle 4 at the entry of a mixing chamber 34 and controlling a valve 40mounted in the pipe 10, assures the maintenance in the chamber 9 of apressure substantially equal to that present at the outlet 33 of thenozzle 4, in the mixing chamber 34.

With the device according to the invention it is possible to obtain aconstant ratio of two gases in a mixture of the latter while maintaininga perfect laminar flow of the superposed coaxial layers of these twogases in the device, in particular at the time when the latter are incontact with one another so that no transmission of energy, within theframework of Bernoullis law, takes place in the device, the mixture ofthe two gases only taking place in the mixing chamber 34 in which aremounted, for example, two inverted pitch helical screws which have notbeen shown on the drawings.

Thus for a mixture of combustible and comburent gas, when the device ismounted on a burner, the ratio of the two gases is not influenced by theback-pressure at the burner outlet.

The coefficient of friction on the walls of the annular space 8supplying the gas originating from the pipe around the jet of the othergas escaping from the nozzle 1, is maintained as constant as possibleindependently of the rate of the gas passing therein when the flowvarres.

It is, however, well known that as a result of the friction of the gason the walls, the rate of flow in the thickness of the jet is notuniform but assumes the shape of a parabola which is the more drawn outthe more the mean rate of the flow increases. It results, therefrom,consequently, that if precautions are not taken, the flow will not beproportionate to the loss of head in this annular space 8. Thus, forthis reason, the outer circular flange 22 has been provided at theoutlet 7 of the first nozzle. The stationary film 39 hereinbeforedescribed, which is then established, substantially reduces thecoefficient of the said friction within the annular space 8 on the outerwall of the nozzle 1 because the coefficient of friction between astationary layer of a fluid and a mobile layer of the same fluid is farless than that for the same fluid on a solid wall.

A similar flange can, of course, be provided on the inner wall of theentry 6 of the nozzle 4. In the annular space 8 between the nozzles 1and 4 a substantially plane cross-sectional diagram of the rates willthen be obtained the form of which, in practice, does not vary withinthe usual limits of flow of the device, thus ensuring the ratio of thetwo gases remaining constant.

On the other hand, another condition met by the device according to theinvention and which is also essential in order to reach the aim,hereinbefore defined, is that it permits to carry out with a very greataccuracy an axial movement of one of the nozzles, under thecircumstances the nozzle 1, with reference to the other nozzle in orderto adjust the ratio of the two gases. This accuracy permits themaintenance, for a relative position of the two nozzles, of a welldefined ratio constant in time for such poition.

In certain known types of devices of this kind, the nozzle 1 is slightlyinclined during its movement so that its axis no longer coincides withthat of the other nozzle 4. The annular space 8, therefore, will nolonger be in that case centered for such devices and the asymmetry thenestablished will hinder the flow of the gas and influence the said ratiowhen the flow varies.

It is for this reason that a particularly advantageous embodiment of thedevice is that in which the drive of the movable nozzle takes place inpoints symmetrical with reference to the axis, for example in locationsdiametrically facing one another.

FIG. 3 shows a modified form of the device hereinbefore described andenables carrying out the movement of the mobile nozzle in a continuousmanner. This embodiment is particularly of interest in order to maintaina constant chemical composition of the mixture of two gases, when thetemperature of one at least of the gases varies. Indeed, the variationof the temperature of a gas results in a variation of its density and,consequently, the number of moles passing through the nozzle 1 under theinfluence of the difference in pressure at the entry of nozzle 1 withreference to the outlet thereof.

For this purpose, according to the invention, the ring 1 l is moved bymeans of a pneumatic or hydraulic servo-motor, not shown in FIG. 3,controlled by an analyser of the gas mixture present at the outlet ofthe nozzle 4, in the mixing chamber 34, this analyser not having beenshown either in FIG. 3.

The outer cylindrical surface of the ring 11 is grooved along itsgeneratrices. The servo-motor drives a straight tooth pinion 37 meshingwith the grooves of the said cylindrical surface, which thus provideteeth.

In normal operational position, the cylindrical portion 30 of the nozzle1 engages nozzle 4 as far as the conical frustrum 28 of the latter asshown in dash lines in FIG. 1. Thus the device does not operate as aninjector as, on account of the laminar and parallel flow of the twogases, the doses of which are to be controlled, there is no exchange ofenergy between such gases, i.e. a drawing of one gas by the other.

It should also be noted that the length of the whole of the deviceaccording to the invention, does not vary when the distance between thetwo nozzles is changed so that its mounting in a supply circuit of anapparatus, for example of a burner, does not present any difficulty.

It is well understood that the invention is not limited to the describedembodiment and that many changes may be introduced therein withoutdeparting from the scope of the present patent application.

Thus either the nozzle 1 or the nozzle 4 and even, as the case may be,both of them may be mobile; that the device may be used also forcontrolling a mixture of any two gases such as hydrogen and air, or ofair and a combustible gas.

On the other hand, the ring 11 may, in a modified form of the object ofthe present invention, be freely threaded on the sleeve 12, the drivingmembers 14 and 15 then engaging the threads of the inner screw thread 24provided in the ring 11. In this modified form, the ring 11 thereforedoes not move along the axis thereof, when it is made to rotate aroundthe latter in order. to adjust the position of the nozzle 1 withreference to the nozzle 4.

I claim:

1. A device for controlling a mixture of two gases, said devicecomprising:

a first nozzle adapted to communicate with a first adduction pipe at theupstream end thereof and having a converging portion at the downstreamend thereof;

a second nozzle coaxial with said first nozzle and having a divergingportion at the upstream end thereof at least the entry of which is oflarger internal cross-section than the external cross-section of thedownstream portion of said first nozzle, whereby said downstream portionof said first nozzle may be inserted in said entry of said secondnozzle;

a housing defining a chamber which is adapted to communicate with asecond adduction pipe, said chamber being shaped so as to enclose thedownstream end of said first nozzle and so as to communicate with theentry of said second nozzle; and

means for varying the relative position of said first and second nozzlesalong their common axis, said means comprising:

a stationary sleeve (a) provided with an external screw thread, (b)enclosing one of said nozzles in a manner permitting said one of saidnozzles to translate inside said stationary sleeve along the axisthereof, and having at least one slot therein which is parallel to saidaxis and which passes right through said sleeve;

at least one ring provided with an internal screw thread, by which it isthreadedly interconnected with the external screw thread on saidstationary sleeve so as to be movable thereon over said slot in saidstationary sleeve, said ring being further provided with an annularrecess coaxial with said sleeve and located on the inner face of saidring; and

a driving member for said one of said nozzles which cooperates with saidslot, is secured to the outer wall of said one of said nozzles, andengages freely said annular recess in said ring, whereby said one ofsaid nozzles can be caused to move axially inside the device by makingsaid ring rotate around its axis.

2. A device as claimed in claim 1 wherein said sleeve has at least twosuch slots located diametrically opposite one another in said sleeve anda like number of driving members for said one of said nozzlescooperating with said slots.

3. A device as claimed in claim 1 wherein said sleeve has a plurality ofsuch slots symmetrically located in said sleeve and a like number ofdriving members for said one of said nozzles cooperating with saidslots.

4. A device for controlling a mixture of two gases, said devicecomprising:

a first nozzle adapted to communicate with a first adduction pipe at theupstream end thereof and having a converging portion at the downstreamend thereof;

a second nozzle coaxial with said first nozzle and having a divergingportion at the upstream end thereof at least the entry of which is oflarger internal cross-section than the external cross-section of thedownstream portion of said first nozzle, whereby said downstream portionof said first nozzle may be inserted in said entry of said secondnozzle;

a housing defining a chamber which is adapted to communicate with asecond adduction pipe, said chamber being shaped so as to enclose thedownstream end of said first nozzle and so as to communicate with theentry of said second nozzle; and

means for varying the relative position of said first and second nozzlesalong their common axis, said means comprising:

a stationary sleeve (a) provided with an external screw thread, (b)enclosing one of said nozzles in a manner permitting said one of saidnozzles to translate inside said stationary sleeve along the axisthereof, and (0) having at least one slot therein which is parallel tosaid axis and which passes right through said sleeve;

a first ring provided with an internal screw thread, by which it isthreadedly interconnected with the external screw thread on saidstationary sleeve so as to be movable thereon over said slot in saidstationary sleeve, said first ring being further provided with agraduation on the visible face thereof showing the ratio of the twogases to be mixed in terms of the position of said first ring on saidsleeve, said graduation extending close to one of the rims of saidsleeve over at least a portion of the periphery thereof;

a second ring mounted in a movable manner on said stationary sleeveclose to said first ring, whereby the location of said second ring maybe adjusted with reference to said first ring, said second ring beingprovided with a mark line extending parallel to the axis of said sleeveon the visible face thereof; and

a driving member for said one of said nozzles which 7 8 the with saidslot, is secured to the outer wall of said cross-section than theexternal cross-section of the one of said nozzles and engages freely theinner downstream portion of said first nozzle, whereby face of saidfirst ring, said downstream portion of said first nozzle may be wherebysaid one of said nozzles can be caused to move inserted in said entry ofsaid second nozzle; axially inside the device by making said ring rotatea housing defining a chamber which is adapted to around its axis.communicate with a second adduction pipe, said 5. A device as claimed inclaim 4 wherein chamber being shaped so as to enclose the downthe outerdiameter of at least the portion of said secstream end of said nozzle toa degree at least suffiond ring adjacent to said first ring is less thatthe cient to include said stationary gas film and so as inner diameterof at least the portion of said first 10 to communicate with the entryof said second nozring adjacent to said second ring, thereby permitzle;and ting at least a portion of said first ring to pass over means forvarying the relative position of said first at least a portion of saidsecond ring, and and second nozzles along their common axis, said saidmark line on said second ring has in its turn gradmeans comprising:

uations providing a graduated scale, the space bea stationary sleeve (a)provided with an external tween two consecutive graduations of suchscale screw thread, (b) enclosing one of said nozzles being equal to thepitch of the internal thread of in a manner permitting said one of saidnozzles said first ring, whereby the latter moves by one to translateinside said stationary sleeve along the graduation on the second ringwhen it completes axis thereof, and (c) having at least one slot onerevolution around its axis. therein which is parallel to said axis andwhich 6. A device for controlling a mixture of two gases, passes rightthrough said sleeve; said device comprising: at least one ring providedwith an internal screw a first nozzle adapted to communicate with afirst adthread, by which it is threadedly interconnected duction pipe atthe upstream end thereof and havwith the external screw thread on saidstationary ing a converging portion at the downstream end sleeve so asto be movable thereon over said slot thereof, said first nozzle beingfurther provided in said stationary sleeve; and with an outer flangearound the outlet thereof the a driving member for said one of saidnozzles which upstream wall of which forms an angle with the cooperateswith said slot, is secured to the outer outside wall of said firstnozzle which is less than or wall of said one of said nozzles, andengages equal to 90", whereby it establishes a stationary gas freely theinner face of said first ring, film extending upstream from said flangeover at whereby said nozzle can be caused to move axially inleast aportion of the outside wall of said first nozside the device by makingsaid ring rotate around its zle when the device is in use; axis. asecond nozzle coaxial with said first nozzle and hav- 7. A device asclaimed in claim 6 wherein said uping a diverging portion at theupstream end thereof stream wall of said flange is planar.

at least the entry of which is of larger internal

1. A device for controlling a mixture of two gases, said devicecomprising: a first nozzle adapted to communicate with a first adductionpipe at the upstream end thereof and having a converging portion at thedownstream end thereof; a second nozzle coaxial with said first nozzleand having a diverging portion at the upstream end thereof at least theentry of which is of larger internal cross-section than the externalcross-section of the downstream portion of said first nozzle, wherebysaid downstream portion of said first nozzle may be inserted in saidentry of said second nozzle; a housing defining a chamber which isadapted to communicate with a second adduction pipe, said chamber beingshaped so as to enclose the downstream end of said first nozzle and soas to communicate with the entry of said second nozzle; and means forvarying the relative position of said first and second nozzles alongtheir common axis, said means comprising: a stationary sleeve (a)provided with an external screw thread, (b) enclosing one of saidnozzles in a manner permitting said one of said nozzles to translateinside said stationary sleeve along the axis thereof, and (c) having atleast one slot therein which is parallel to said axis and which passesright through said sleeve; at least one ring provided with an internalscrew thread, by which it is threadedly interconnected with the externalscrew thread on said stationary sleeve so as to be movable thereon oversaid slot in said stationary sleeve, said ring being further providedwith an annular recess coaxial with said sleeve and located on the innerface of said ring; and a driving member for said one of said nozzleswhich cooperates with said slot, is secured to the outer wall of saidone of said nozzles, and engages freely said annular recess in saidring, whereby said one of said nozzles can be caused to move axiallyinside the device by making said ring rotate around its axis.
 2. Adevice as claimed in claim 1 wherein said sleeve has at least two suchslots located diametrically opposite one another in said sleeve and alike number of driving members for said one of said nozzles cooperatingwith said slots.
 3. A device as claimed in claim 1 wherein said sleevehas a plurality of such slots symmetrically located in said sleeve and alike number of driving members for said one of said nozzles cooperatingwith said slots.
 4. A device for controlling a mixture of two gases,said device comprising: a first nozzle adapted to communicate with afirst adduction pipe at the upstream end thereof and having a convergingportion at the downstream end thereof; a second nozzle coaxial with saidfirst nozzle and having a diverging portion at the upstream end thereofat least the entry of which is of larger internal cross-section than theexternal cross-section of the downstream portion of said first nozzle,whereby said downstream portion of said first nozzle may be inserted insaid entry of said second nozzle; a housing defining a chamber which isadapted to communicate with a second adduction pipe, said chamber beingshaped so as to Enclose the downstream end of said first nozzle and soas to communicate with the entry of said second nozzle; and means forvarying the relative position of said first and second nozzles alongtheir common axis, said means comprising: a stationary sleeve (a)provided with an external screw thread, (b) enclosing one of saidnozzles in a manner permitting said one of said nozzles to translateinside said stationary sleeve along the axis thereof, and (c) having atleast one slot therein which is parallel to said axis and which passesright through said sleeve; a first ring provided with an internal screwthread, by which it is threadedly interconnected with the external screwthread on said stationary sleeve so as to be movable thereon over saidslot in said stationary sleeve, said first ring being further providedwith a graduation on the visible face thereof showing the ratio of thetwo gases to be mixed in terms of the position of said first ring onsaid sleeve, said graduation extending close to one of the rims of saidsleeve over at least a portion of the periphery thereof; a second ringmounted in a movable manner on said stationary sleeve close to saidfirst ring, whereby the location of said second ring may be adjustedwith reference to said first ring, said second ring being provided witha mark line extending parallel to the axis of said sleeve on the visibleface thereof; and a driving member for said one of said nozzles whichthe with said slot, is secured to the outer wall of said one of saidnozzles and engages freely the inner face of said first ring, wherebysaid one of said nozzles can be caused to move axially inside the deviceby making said ring rotate around its axis.
 5. A device as claimed inclaim 4 wherein the outer diameter of at least the portion of saidsecond ring adjacent to said first ring is less that the inner diameterof at least the portion of said first ring adjacent to said second ring,thereby permitting at least a portion of said first ring to pass over atleast a portion of said second ring, and said mark line on said secondring has in its turn graduations providing a graduated scale, the spacebetween two consecutive graduations of such scale being equal to thepitch of the internal thread of said first ring, whereby the lattermoves by one graduation on the second ring when it completes onerevolution around its axis.
 6. A device for controlling a mixture of twogases, said device comprising: a first nozzle adapted to communicatewith a first adduction pipe at the upstream end thereof and having aconverging portion at the downstream end thereof, said first nozzlebeing further provided with an outer flange around the outlet thereofthe upstream wall of which forms an angle with the outside wall of saidfirst nozzle which is less than or equal to 90*, whereby it establishesa stationary gas film extending upstream from said flange over at leasta portion of the outside wall of said first nozzle when the device is inuse; a second nozzle coaxial with said first nozzle and having adiverging portion at the upstream end thereof at least the entry ofwhich is of larger internal cross-section than the externalcross-section of the downstream portion of said first nozzle, wherebysaid downstream portion of said first nozzle may be inserted in saidentry of said second nozzle; a housing defining a chamber which isadapted to communicate with a second adduction pipe, said chamber beingshaped so as to enclose the downstream end of said nozzle to a degree atleast sufficient to include said stationary gas film and so as tocommunicate with the entry of said second nozzle; and means for varyingthe relative position of said first and second nozzles along theircommon axis, said means comprising: a stationary sleeve (a) providedwith an external screw thread, (b) enclosing one of said nozzles in amanner permitting said one of said nozzles to translate inside saiDstationary sleeve along the axis thereof, and (c) having at least oneslot therein which is parallel to said axis and which passes rightthrough said sleeve; at least one ring provided with an internal screwthread, by which it is threadedly interconnected with the external screwthread on said stationary sleeve so as to be movable thereon over saidslot in said stationary sleeve; and a driving member for said one ofsaid nozzles which cooperates with said slot, is secured to the outerwall of said one of said nozzles, and engages freely the inner face ofsaid first ring, whereby said nozzle can be caused to move axiallyinside the device by making said ring rotate around its axis.
 7. Adevice as claimed in claim 6 wherein said upstream wall of said flangeis planar.